Conventionally, it has been known that if the injection pressure rises too sharply in an early stage of fuel combustion, engine knocking tends to occur. Therefore it is possible to reduce the combustion noises of diesel engines by controlling the rate of fuel injection in the early stage of fuel injection. This can be achieved by injecting fuel in two-stages.
In a typical fuel injection nozzle, the needle valve is biased toward the closing direction by a pressure spring and is lifted against the spring force of the pressure spring by the pressure from the fuel. By using two pressure springs, it is possible to control the fuel pressure vs. needle valve lift curve, but the structure of the nozzle assembly becomes excessively complex.
Japanese utility model laid open publication No. 59-17268 discloses a needle valve for a fuel injection nozzle which is provided with a port and passage which controls the back pressure acting upon the needle valve according to its lift in cooperation with a fixed part of the nozzle body. However, provision of such a spool valve structure at the upper end of the needle valve involve an increase in the manufacturing cost.
Japanese patent laid open publication No. 58-204962 discloses an electronically controlled fuel injection system. Each nozzle assembly is provided with a pair of solenoid valves, and they must be controlled with a complex control unit. Thus, this also involves high cost and complexity of structure.